1. Field of the Invention
This invention relates to lens systems of short total length measured from the front vertex to the focal plane, and more particularly to a small photographic objective with an angular field of about 63.degree. and an F-number of about 3.5.
2. Description of the Prior Art
Recently, as cameras have been reduced in size, the demand has arisen for compact lenses of short focal length. Where the total length of a lens system is reduced to less than the focal length, it is desirable to distribute positive refractive power over the front group of lenses in the lens system and a negative refractive power over the rear group of lenses, or to employ a so-called telephoto-type system. Such power distribution has found many uses in long focal length lenses of narrow angular fields, but there are only a few examples of its application to wide angle objectives having angular fields of at least 60.degree. in a short lens system with a large relative aperture.
With this type of power distribution, when the total length is reduced, and when the relative aperture is increased also, while the angular field is also widened, the distortion and astigmatism deteriorate badly or extremely large increases of coma and halo result.
U.S. Pat. No. 3,998,527 and Japanese Pat. Sho 44-10831 illustrate attempts to provide for photographic objectives of the telephoto type with increased angular fields. In the Japanese patent, the angular field is attained at a relatively narrow value, namely 46.degree., while the U.S. Patent illustrates a relatively wide angular field, namely 63.degree., but results in an F-number of 4.5.